5708
financial support and to Professor H. Taube, Stanford University, for encouragement, (12) Max ICade Foundation, Inc., Research Fellow, 1967-1968. (13) National Science Foundation Postdoctoral Fellow, 1966-1967. (14) National Institutes of Health Postdoctoral Fellow, 1966-1967.
E. E. van Tamelen, Gernot Boche,I* S. W. Ela,I3 R. B. Fechter14 Department of Chemistry, Statiford Unicersity Stanford, California 94305 Received September 11, I967
A Facile Reaction of Organoboranes with Methyl Vinyl Ketone. A Convenient New Ketone Synthesis uia Hydroboration Sir .I We wish to report that trialkylboranes, readily available cia hydroboration, * undergo a remarkably fast 1,4 addition to methyl vinyl ketone. Hydrolysis of the initially formed intermediate produces the corresponding methyl ketone I. CH3
R3B
I +RCHzCH=C-OBR?
+ CHFCHCOCHB
4
HzO
RCHzCHzCOCHa
I
Indeed, both the addition and hydrolysis can be made to occur concurrently in a few minutes at 25 ”, providing a remarkably simple conversion of a large variety of olefins into the corresponding ketones 11-VIII. Since
to make the synthesis broadly applicable to a wide variety of structures and derivatives. We have utilized two general procedures. Procedure A. In a 200-ml flask was placed 11.2 g (100 mmoles) of 1-octene and 30 ml of tetrahydrofuran. The flask was flushed with nitrogen and 3.3 ml of a 1.00 M solution of borane in tetrahydrofuran was injected with a hypodermic syringe to achieve hydrob ~ r a t i o n . ~After 1 hr, 3.5 g (50 mmoles) of methyl vinyl ketone in 15 ml of tetrahydrofuran was added and the reaction mixture was maintained at 40” for 2 hr. Then 15 ml of water was added and the solution was heated at 40” for 1 hr to hydrolyze the enol dialkylborinite intermediate. The solution was dried over magnesium sulfate. Glpc analysis indicated the formation of 31 moles of ketone, a yield of 9 3 z , containing 2-decanone and 5-methyl-2-undecanone in an 86 : 14 ratio. Procedure B. Hydroboration was carried out as above, using 120 mmoles of 1-octene, to form 40 mmoles of the organoborane. Then 1.8 ml (100 mmoles) of water was added, followed by 4.4 g (60 mmoles) of methyl vinyl ketone. The reaction mixture was allowed to stir for 1 hr at 25°.5 Glpc analysis indicated the presence of 39.5 mmoles of the dodecanones, a yield of 99 %, in a molar ratio of 85 :15. The ketones were isolated by removal of the tetrahydrofuran under vacuum followed by a simple distillation of the reaction mixture. No difficulty was encountered in the separation from the much higher boiling boron-containing side products. The experimental results are summarized in Table I.
C H I C H ~ C H = C H ?+CHa(CH2)jCOCHa 11
CHaCH=CHCH3
Table I. Conversion of Olefins into 4-Alkyl-2-butanones by
+CH~CH~CH(CH,)CH?CHZCOCH~ Reaction of the Corresponding Organoboranes with Methyl Vinyl Ketone
111 (CH,)zC=CHz
----f
CH3(CHz)jCH=CHz
Q
f
0-
-
(CH~)ZCH(CH~)~COCH~ IV CHz(CH?)gCOCH3 V
Olefin
Product,a
2-Hexanone Ethylene 2-Heptanoneb Propylene 1-Butene 2-Octanone, 85 5-Methyl-2-heptanone, 15 2-Butene 5-Methyl-2-heptanone 6-Methyl-2-heptanone Isobutylene I-Octene 2-Dodecanone, 85 5-Methyl-2-undecanone, 15 6-Phenyl-2-butanone, 57 Styrene 5-Methyl-j-phenyl-2pentanone, 43 Cyclopentene 4-Cyclopentyl-2- butanone Cyclohexene 4-Cyclohexyl-2-butanone Norbornene .l-(exo-Norborny1)-2butanone
VCH,CH:COCH, VI o/CHCH&OCH, VI1
&,cH2cH2cocHJ VI11
methyl ketones are readily converted into the corresponding acids by the haloform reaction, * this likewise provides a simple synthesis of the corresponding carboxylic acids involving a lengthening of the chain by three carbon atoms. There is a disadvantage in that only one of the three groups on boron participates in the reaction under these mild conditions. However, this is compensated for by the rcmarkable speed with which the reaction occurs and the mildness of the conditions which promise (1) H . C. Brown, “Hydroboration,” W. A. Benjamin, Inc., New York, N. Y . , 1962. (2) R. C. Fuson and B. A. Bull. Chem. Rea., 15, 275 (1934).
Journal of the American Chemical Society
1 89:22 /
Yield;.d Procedure Procedure A B 99
100 99 (86) 80 (70) 65 (50)e 93 (85.5)
99 (83)
93
100
99 (86) 95 (80) 99 (80)
All products were either compared with authentic samples or exhibited analytical data and spectra in accordance with the assigned structures. * Analysis for the two isomers was n o t made. c By glpc analysis. d Numbers in parentheses a r e isolated yields. e Reaction relatively slow; yield increased with additional time. Q
Hydroboration of a 1-alkene produces approximately 6 % of the secondary alkyl derivative. Hydroboration (3) G. Zweifel and H. C. Brown, Org. Rencfions, 13, I (1963). (4) Clathration with urea provides a simple means of separating the major product from the branched-chain impurity. ( 5 ) Actually, glpc examination indicated that the reaction was essentially complete in 10 min.
October 25, 1967
5709
of styrene produces 18 % of the secondary derivative. Consequently, the formation of 14 % 5-methyl-2-undecanone from I-octene and 43 % 5-methyl-5-phenyl2-pentanone indicates that secondary groups have a large preference for participating in the migration as compared to the corresponding primary groups. The precise mechanism of this unusually facile reaction has many points of interest, and we plan to explore it in the future. However, at the moment it appears more urgent to establish the full scope of the reaction. It is evident that many a,@-unsaturated ketones6 should be capable of participating in this reaction, as well as many other vinyl derivatives.' Consequently, we are devoting our main efforts at the present time to a broad exploration of the full scope of this interesting reaction. It is becoming evident that the organoboranes are exceedingly versatile reactants, with major possibilities in organic synthesis. Among the more interesting possibilities to which attention has already been called are : isomerization, displacement, contrathermodynamic isomerization of olefins, cyclization*"Ib protonolysis, oxidation to alcohols (alkaline hydrogen peroxide), oxidation to ketones (chromic acid),8c amination,8d metallation,*e coupling with alkaline silver nitrate, homologation,8f and carbonylation8g to tertiary alcohols,8h ketones,*' methylol derivatives,8j acids,8k ring ketones,81 and polycyclics.*" The development described in the present publication opens the door to a new area with major promise. (6) The present reaction bears a number of points of resemblance to the "reductive alkylation of benzoquinone" described by M. F. Hawthorne and M. Reintjes, J . Am. Chem. SOC.,87, 4585 (1965). (7) Actually, we have observed that trialkylboranes react with acrolein with extraordinary speed to form the corresponding aldehydes: H. C. Brown, M. RogiC, M. W. Rathke, and G . W. Kabalka, ibid., 89, 5709 (1067). (8) (a) H. C. Brown, I
